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PPP, PPPoL2TP: properly ack receipt of duplicate packets

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Managed to find the spirit behind the RFC. Looks like we need to send
a ZLB packet with counters as is to the packet (ZLB or not) we
previously sent to ack the message. Luckily we don't need more than
received NS/NR counters to forge the resent ack.

Signed-off-by: Sylvain Rochet <gradator@gradator.net>
This commit is contained in:
Sylvain Rochet 2018-06-15 17:29:50 +02:00
parent 738a2fe846
commit f31160a6cb
1 changed files with 15 additions and 7 deletions
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22
src/netif/ppp/pppol2tp.c
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@ -85,7 +85,7 @@ static err_t pppol2tp_send_sccrq(pppol2tp_pcb *l2tp);
static err_t pppol2tp_send_scccn(pppol2tp_pcb *l2tp, u16_t ns);
static err_t pppol2tp_send_icrq(pppol2tp_pcb *l2tp, u16_t ns);
static err_t pppol2tp_send_iccn(pppol2tp_pcb *l2tp, u16_t ns);
static err_t pppol2tp_send_zlb(pppol2tp_pcb *l2tp, u16_t ns);
static err_t pppol2tp_send_zlb(pppol2tp_pcb *l2tp, u16_t ns, u16_t nr);
static err_t pppol2tp_send_stopccn(pppol2tp_pcb *l2tp, u16_t ns);
static err_t pppol2tp_xmit(pppol2tp_pcb *l2tp, struct pbuf *pb);
static err_t pppol2tp_udp_send(pppol2tp_pcb *l2tp, struct pbuf *pb);
@ -499,9 +499,17 @@ static void pppol2tp_dispatch_control_packet(pppol2tp_pcb *l2tp, u16_t port, str
if (ns != l2tp->peer_ns) {
PPPDEBUG(LOG_DEBUG, ("pppol2tp: drop unexpected packet: received NS=%d, expected NS=%d\n", ns, l2tp->peer_ns));
/*
* The RFC says we must send a ZLB ack for duplicate packets but preventing an endless
* ZLB packets loop came out wayyyy more complicated than just dropping the packet.
* In order to ensure that all messages are acknowledged properly
* (particularly in the case of a lost ZLB ACK message), receipt
* of duplicate messages MUST be acknowledged.
*
* In this very special case we Ack a packet we previously received.
* Therefore our NS is the NR we just received. And our NR is the
* NS we just received plus one.
*/
if ((s16_t)(ns - l2tp->peer_ns) < 0) {
pppol2tp_send_zlb(l2tp, nr, ns+1);
}
return;
}
@ -563,7 +571,7 @@ static void pppol2tp_dispatch_control_packet(pppol2tp_pcb *l2tp, u16_t port, str
break;
/* Stop Control Connection Notification */
case PPPOL2TP_MESSAGETYPE_STOPCCN:
pppol2tp_send_zlb(l2tp, l2tp->our_ns+1); /* Ack the StopCCN before we switch to down state */
pppol2tp_send_zlb(l2tp, l2tp->our_ns+1, l2tp->peer_ns); /* Ack the StopCCN before we switch to down state */
if (l2tp->phase < PPPOL2TP_STATE_DATA) {
pppol2tp_abort_connect(l2tp);
} else if (l2tp->phase == PPPOL2TP_STATE_DATA) {
@ -714,7 +722,7 @@ nextavp:
return;
send_zlb:
pppol2tp_send_zlb(l2tp, l2tp->our_ns+1);
pppol2tp_send_zlb(l2tp, l2tp->our_ns+1, l2tp->peer_ns);
return;
packet_too_short:
PPPDEBUG(LOG_DEBUG, ("pppol2tp: packet too short: %d\n", p->len));
@ -1042,7 +1050,7 @@ static err_t pppol2tp_send_iccn(pppol2tp_pcb *l2tp, u16_t ns) {
}
/* Send a ZLB ACK packet */
static err_t pppol2tp_send_zlb(pppol2tp_pcb *l2tp, u16_t ns) {
static err_t pppol2tp_send_zlb(pppol2tp_pcb *l2tp, u16_t ns, u16_t nr) {
struct pbuf *pb;
u8_t *p;
u16_t len;
@ -1065,7 +1073,7 @@ static err_t pppol2tp_send_zlb(pppol2tp_pcb *l2tp, u16_t ns) {
PUTSHORT(l2tp->source_tunnel_id, p); /* Tunnel Id */
PUTSHORT(0, p); /* Session Id */
PUTSHORT(ns, p); /* NS Sequence number - to peer */
PUTSHORT(l2tp->peer_ns, p); /* NR Sequence number - expected for peer */
PUTSHORT(nr, p); /* NR Sequence number - expected for peer */
return pppol2tp_udp_send(l2tp, pb);
}